Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 240114

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
914 PM EDT MON MAY 23 2016

Upper level low pressure off the Mid-Atlantic coast this evening
will move northeastward towards New England overnight through
Tuesday and then toward the Canadian Maritimes Tuesday night.
High pressure will build overhead Wednesday before settling over
the western Atlantic Thursday through Monday.


Upper-level low pressure continues to spin just off the Delmarva
Peninsula this evening. Convection this afternoon has dissipated
due to the loss of daytime heating. However...shortwave energy
aloft continues to rotate around the upper-level low so clouds and
a few showers will continue through this evening. Coverage will be
isolated or widely scattered since instability has diminished.

The upper-level low will gradually shift northeast toward New
England overnight. Drier air will move in from the north behind
this system...allowing for clearing skies and mainly dry
conditions overnight. An isolated shower cannot be ruled out east
of the Blue Ridge Mountains...but any precipitation will be light
and most of the time will be dry. Will see lows in the 50s for
most, with some 40s north/west.


Upper low will continue pulling away on Tuesday, and we will see
drier air wrapping in behind it. This should lead to more sun than
today with some scattered instability cumulus and some cirrus.
Could also see an isolated instability shower or sprinkle, but
coverage will be spotty at best. Temperatures will be much warmer
than recent days with highs in the upper 70s to around 80 degrees.

Upper ridge builds into the region on Wednesday ushering in more
summer-like weather. The surface high will slide south of the
region, allowing for a warm southerly flow to develop. Wednesday
should remain dry for much of the region with highs in the 80s
area-wide. With dew points hovering in the 50s, should be quite
nice, with relatively low humidity. Not expecting much in the way
of shower/thunderstorm activity either, except for an isolated
one or two possible in the mountains as a weak ripple moves
through aloft.


Summer-like pattern begins Thursday, with Bermuda high promoting
warm/moist advection across the Mid-Atlantic. Increasing moisture
will lead to typical summertime diurnal thunderstorm activity
Thursday and Friday, with the best chances over the higher terrain
and fairly low chances east of the Blue Ridge. While instability
will be in the moderate range during peak heating, rather anemic
wind field keeps effective bulk shear generally less than 20 kts,
suggesting pulse-type thunderstorms. Highs Thursday and Friday
generally in the M/U80s, though a few 90s are possible, especially
in areas bereft of thunderstorm activity.

Mid-level heights begin to rise Saturday, which leads to poor
mid- level lapse rates. As such, guidance has really backed off on
QPF Saturday.

Model spread increases by Sunday, so will take a blended approach
for now until better agreement can be reached.


VFR at most sites at most times through this evening. Recent
rainfall and light winds along with clearing skies will allow for
patchy fog to develop. Do feel that there should be just enough of
a gradient wind to keep widespread fog from developing.
However...MVFR vsbys are likely overnight. IFR conditions cannot
be ruled out...but confidence is to low to put in the forecast at
this time due to the light gradient wind.

VFR returns Tuesday and should persist for most
through at least Wednesday night.

Winds will be light (less than 10 knots) and generally
west/northwesterly through tonight and into Tuesday. Some gusts up
to about 15 knots likely Tuesday afternoon. Light southwest flow
then develops by Wednesday.

While most of Thursday through Saturday will end up VFR, a few
afternoon thunderstorms are possible.


Winds will shift to the northwest tonight but speeds should remain
below sca criteria. It may be close overnight into early Tuesday
morning for middle portions of the bay and the lower tidal potomac
river...but gusts have been capped at 15 knots for now due to a
light gradient. Winds will continue out of the west/northwest
Tuesday around the departing low with gusts around 15 knots
expected. Southwest winds then develop Wednesday, but should also
remain sub- SCA.

While flow remains sub-SCA, a low chance of an afternoon
thunderstorms exists over the waters Thursday and Friday.


CBOFS and 12 ESTOFS indicates potential for minor coastal flooding
at Annapolis Tuesday morning. ETSS has water levels well below
flooding criteria. Leaned the forecast closer to the ETSS this
evening. Latest levels are running closer to the ETSS...and an
offshore flow is expected with the low moving away from the area
overnight into Tuesday.


We put together some records (preliminary) of how this month compares
to rainy Mays on record for our area.

Washington, DC (records since 1871)
Most days in May with at least a trace: 25 (in 1952
and 1943) So far in 2016 (through the 23rd): 19

Most days in May with at least 0.01": 20 (in 2003)
So far in 2016 (through the 23rd): 18

Highest May monthly rainfall: 10.69" (in 1953 and 1889)
So far in 2016 (through 4 PM on the 23rd): 5.13"

Baltimore, MD (records since 1870)
Most days in May with at least a trace: 25 (in 2003)
So far in 2016 (through the 23rd): 19

Most days in May with at least 0.01": 21 (in 1882)
So far in 2016 (through the 23rd): 15

Highest May monthly rainfall: 8.71" (in 1989)
So far in 2016 (through 4 PM on the 23rd): 4.61"

Dulles, VA
Most days in May with at least a trace: 23 (in 2003)
So far in 2016 (through the 23rd): 20

Most days in May with at least 0.01": 21 (in 2003)
So far in 2016 (through the 23rd): 16

Highest May monthly rainfall: 10.26" (in 2009 and 1988)
So far in 2016 (through 4 PM on the 23rd): 5.62"




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